High moisture-absorbing and moisture-releasing properties have been required for cotton clothing products (cellulose fiber products) such as underwear. To obtain such cotton clothing products (cellulose fiber products) with high moisture-absorbing and moisture-releasing properties, a method for hydrophilic treatment of cellulose fiber, which is used as a starting material, can be used. One typical method for hydrophilic treatment of cellulose fiber among various methods is oxidation of the hydroxyl group in the cellulose into a carboxyl group.
To oxidize the hydroxy group in the cellulose into a carboxyl group, a method for oxidizing a cellulose fiber starting material in a reaction solution containing a halogen-based oxidizing agent and an N-oxyl compound such as 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) is known. Another known method is a method in which dehalogenation to remove halogens remaining in oxidized cellulose is performed, and then reduction to reduce a ketone group at position 2 and/or position 3 of a glucose unit, which is generated by the production of oxidized cellulose fibers, is performed (e.g., Patent Literature 1).